QoS (Quality of Service) is a key problem to be solved urgently in the next generation networks. In the trend of convergence in the next generation network, an IP network will become a unified transport platform for carrying voice, data and video. Since the IP network adopts connectionless-oriented IP protocols and can only provide the best-effort service, the QoS guarantee can not be provided.
QoS means network performance related to the application requirements and certain techniques for guaranteeing the network performance. At present, several mechanisms for realizing QoS guarantee have been proposed.
An IntServ/RSVP (Integrated Service/Resource Reservation Protocol) Model
The RSVP protocol is used to establish resource reservation in an integrated service network. A user terminal sends a QoS request for an application stream to the network using the RSVP protocol. A router receiving the QoS request determines a path of the application stream, transmits the information of the QoS request to other routers in the path, establishes and saves the service information and reserves certain resources for the application stream. The resource reservation is to reserve certain resources for the application stream along the path of the application stream from source end to receiving end. Though this service model can provide absolute QoS guarantee, it has a relatively high requirement which requires the router to support the RSVP protocol and an admission control protocol. The service model also needs to spend a lot of resources to maintain and update databases. It is complicated to implement the model so that the network using the model has poor expansibility.
A DiffServ (Differentiated Service) Model
The basic idea of the DiffServ Model is to mark a Differentiated Service Code Point (DSCP) for a data packet at an entrance of the network to indicate how the data packet is processed at the intermediate node of the forwarding path in the network. Some different service levels can be formed by setting different mark for a DSCP field of the data packet and the processing based on the DSCP field. Thus, at an entrance of a Service Provider's network, an edge router can performs operations such as classifying, marking, policing, etc. The main task of the core network is only to take corresponding actions to forward the data packet, and assign routes to the data packet, based on the DSCP mark in the head of the data packet. Detailed DiffServ mechanism is described in RFC 2475. This DiffServ mechanism is easy to be implemented, that is, it only requires mapping the service request to DSCP at the edge of the network, without taking explicit resource reservation signaling at the core node of the network, thereby decreasing the complexity. However, this model cannot provide absolute guarantee for each stream to have sufficient resources because it has no access control and signaling mechanism.
A MPLS (Multi-Protocol Label Switching) Technique
MPLS is a switching technique combining layer 2 and layer 3, which cannot solve the QoS problem itself but can assist to solve the QoS problem. A label-based mechanism is introduced by MPLS, which decouples route selection from data forwarding and defines a path of a packet passing the network with labels. A MPLS network is composed of a Label Switching Router (LSR) in the core and a Label Edge Routers (LER) in the edge. The function of a LSR can be considered as a combination of an ATM switch and a traditional router, being composed of a control unit and a switching unit. The function of a LER is to analyze an IP packet head to determine corresponding transport level and a label switching path (LSP). MPLS supports the DiffServ Model, being able to map a plurality of BAs (Behavior Aggregation) of the DiffServ to one LSP of MPLS and forward the traffic on the LSP based on the PHB (Per-Hop-Behavior) of the BAs. However, the combination of MPLS and DiffServ is only limited in the processing manner of data plane, it cannot guarantee the QoS of the data stream entering the network.